1. Field of the Invention
The present invention relates to a low pressure mercury vapor discharge lamp having a main amalgam for controlling the mercury vapor pressure in the tube of the lamp except for a starting period and having an auxiliary amalgam for controlling the mercury vapor pressure in the tube of the lamp for the starting period. In particular, the invention relates to a low pressure mercury vapor discharge lamp designed to operate in a relatively low to middle tube wall temperature area.
2. Description of the Related Art
It is well known that an amalgam can control the mercury vapor pressure of a low mercury vapor pressure discharge lamp at a certain proper level even if the lamp has high tube wall temperature and cannot control the mercury vapor pressure without the amalgam (pure mercury). Therefore, the amalgams are used in the lamps which have high tube wall temperature at their operation, for example, compact flourescent lamps which have bent tubes forming convoluted discharge paths. These lamps have high wall loads (more than 500 W/m.sup.2), and thus have high tube wall temperatures of about 80.degree.-120.degree. C. The amalgam used in these lamps, in other words, the amalgam used in the high tube wall temperature area, is usually bismuth-indium-mercury (Bi-In-Hg) amalgam. The lamps using the bismuth-indium-mercury (Bi-In-Hg) amalgams have an advantage in that the mercury vapor pressures of the lamps are maintained at a certain proper level and the luminous outputs are maintained at a high level.
However, these lamps using the bismuth-indium-mercury (Bi-In-Hg) amalgams have a disadvantage in that the increasing speed of luminous flux of lamps using these amalgams in a starting period (a luminous flux increasing characteristic) is low as compared with lamps not using amalgams. The reason is as follows. The mercury vapor pressure of the bismuth-indium-mercury (Bi-In-Hg) amalgam is lower than that of the pure mercury, and thus reaches a proper level in a high tube wall temperature area. But the mercury vapor pressure becomes too low in a low and middle tube wall temperature area of about 0.degree.-50.degree. C., where the mercury vapor pressure of the pure mecury is a proper level. The tube wall temperature is low for the starting period, and thus the mercury vapor pressure of the bismuth-indium-mercury (Bi-In-Hg) amalgam is low for the starting period.
To eliminate the above disadvantage, an auxiliary amalgam is added to the lamp and located near the electrode of the lamp so that the auxiliary amalgam emits mercury for the starting period. Usually, a conventional auxiliary amalgam is indium-mercury (In-Hg) amalgam. The bismuth-indium-mercury (Bi-In-Hg) amalgam for controlling the mercury vapor pressure of the lamp is called a main amalgam in contrast with the auxiliary amalgam. The auxiliary amalgam is heated by the electrode after the ignition of the lamp and emits mercury vapor in the tube of the lamp. The mercury vapor emitted from the auxiliary amalgam makes up for the lack of the mercury vapor in the starting period.
Furthermore, an amalgam applied for the lamp used in the low and middle tube wall temperature area is developed. For example, bismuth-indium-lead-mercury (Bi-In-Pb-Hg) amalgam and bismuth-lead-mercury (Bi-Pb-Hg) amalgam is disclosed in U.S. Pat. No. 4,972,118.
The bismuth-indium-lead-mercury (Bi-In-Pb-Hg) amalgam and the bismuth-lead-mercury (Bi-Pb-Hg) amalgam are used not only for the purpose of controlling the mercury vapor pressure in the tube of the lamp, but also for the purpose of keeping a certain amount of the mercury sealed in the tube of the lamp. The mercury vapor pressures of these amalgams are higher than that of the bismuth-indium-mercury (Bi-In-Hg) amalgam and are lower than that of the pure mercury. The mercury vapor pressures of these amalgams are similar to that of pure mercury in the low and middle tube wall temperature area.
However, when the auxiliary amalgam made of the indium-mercury (In-Hg) amalgam is provided to the lamp having the bismuth-indium-lead-mercury (Bi-In-Pb-Hg) amalgam or the bismuth-lead-mercury (Bi-Pb-Hg) amalgam provided as a main amalgam, the lamp shows the above mentioned disadvantage in that the luminous flux increasing characteristic is almost as bad in the low and middle tube wall temperature area as the lamp not having the auxiliary amalgam. In other words, the lamp having the main amalgam such as the bismuth-indium-lead-mercury (Bi-In-Pb-Hg) amalgam or the bismuth-lead-mercury (Bi-Pb-Hg) amalgam and the auxiliary amalgam such as the indium-mercury (In-Hg) amalgam has an advantage in that the mercury vapor pressure of the lamp is maintained at a certain proper level in the low and middle tube wall temperature area and that the luminus output is maintained at a high level in that temperature area, but this lamp has a disadvantage in that the luminous flux increasing characteristic is bad in the low and middle tube wall temperature area. This lamp has also a disadvantage in that the tube wall of the lamp has dark spots and dark bands near the electrodes early in its life (a blackening phenomena).